Unix Support frequently advises people not to use /bin/csh. Here is the classic document by Tom Christiansen ( tchrist@mox.perl.com) on why you shouldn't use it for scripts.

Csh Programming Considered Harmful

Resolved: The csh is a tool utterly inadequate for programming, and its use for such purposes should be strictly banned.

I am continually shocked and dismayed to see people write test cases, install scripts, and other random hackery using the csh. Lack of proficiency in the Bourne shell has been known to cause errors in /etc/rc and .cronrc files, which is a problem, because you must write these files in that language.

The csh is seductive because the conditionals are more C-like, so the path of least resistance if chosen and a csh script is written. Sadly, this is a lost cause, and the programmer seldom even realizes it, even when they find that many simple things they wish to do range from cumbersome to impossible in the csh.


The most common problem encountered in csh programming is that you can't do file-descriptor manipulation. All you are able to do is redirect stdin, or stdout, or dup stderr into stdout. Bourne-compatible shells offer you an abundance of more exotic possibilities.

Writing Files

In the Bourne shell, you can open or dup random file descriptors. For example,

exec 2>errs.out

means that from then on, stderr goes into errs file.

Or what if you just want to throw away stderr and leave stdout alone? Pretty simple operation, eh?

cmd 2>/dev/null

Works in the Bourne shell. In the csh, you can only make a pitiful attempt like this:

(cmd > /dev/tty) >& /dev/null

But who said that stdout was my tty? So it's wrong. This simple operation cannot be done in the csh.

Reading Files

In the csh, all you've got is $<, which reads a line from your tty. What if you've redirected stdin? Tough noogies, you still get your tty, which you really can't redirect. Now, the read statement in the Bourne shell allows you to read from stdin, which catches redirection. It also means that you can do things like this:

exec 3<file1
exec 4<file2

Now you can read from fd 3 and get lines from file1, or from file2 through fd 4. In modern, Bourne-like shells, this suffices:

read some_var 0<&3
read another_var 0<&4

Although in older ones where read only goes from 0, you trick it:

exec 5<&0 # save old stdin
exec 0<&3; read some_var
exec 0<&4; read another_var
exec 0<&5 # restore it

Closing FDs

In the Bourne shell, you can close file descriptors you don't want open, like 2>&-, which isn't the same as redirecting it to /dev/null.

More Elaborate Combinations

Maybe you want to pipe stderr to a command and leave stdout alone. Not too hard an idea, right? You can't do this in the csh as I mentioned in 1a. In a Bourne shell, you can do things like this:

exec 3>&1; grep yyy xxx 2>&1 1>&3 3>&- | sed s/file/foobar/ 1>&2 3>&-
grep: xxx: No such foobar or directory

Normal output would be unaffected. The closes there were in case something really cared about all its FDs. We send stderr to sed, and then put it back out 2.

Consider the pipeline:

A | B | C

You want to know the status of C, well, that's easy: it's in $?, or $status in csh. But if you want it from A, you're out of luck -- if you're in the csh, that is. In the Bourne shell, you can get it, although doing so is a bit tricky. Here's something I had to do where I ran dd's stderr into a grep -v pipe to get rid of the records in/out noise, but had to return the dd's exit status, not the grep's:

dd_noise='^[0-9]+\+[0-9]+ records (in|out)$'
exec 3>&1
status=`((dd if=$device ibs=64k 2>&1 1>&3 3>&- 4>&-; echo $? >&4) |
egrep -v "$dd_noise" 1>&2 3>&- 4>&-) 4>&1`
exit $status;



The csh is a horrid botch with its built-ins. You can't put them together in many reasonable way. Even simple little things like this:

% time | echo

which while nonsensical, shouldn't give me this message:

Reset tty pgrp from 9341 to 26678

Others are more fun:

% sleep 1 | while
while: Too few arguments.
[5] 9402
% jobs
[5] 9402 Done sleep |

Some can even hang your shell. Try typing ^Z while you're sourcing something, or redirecting a source command. Just make sure you have another window handy.

Flow control

You can't mix flow-control and commands, like this:

who | while read line; do
echo "gotta $line"

You can't combine multiline constructs in a csh using semicolons. There's no easy way to do this

alias cmd 'if (foo) then bar; else snark; endif'

Stupid parsing bugs

Certain reasonable things just don't work, like this:

% kill -1 `cat foo`
`cat foo`: Ambiguous.

But this is ok:

% /bin/kill -1 `cat foo`

If you have a stopped job:

[2] Stopped rlogin globhost

You should be able to kill it with

% kill %?glob
kill: No match

% fg %?glob


In the csh, all you can do with signals is trap SIGINT. In the Bourne shell, you can trap any signal, or the end-of-program exit. For example, to blow away a tempfile on any of a variety of signals:

$ trap 'rm -f /usr/adm/tmp/i$$ ;
echo "ERROR: abnormal exit";
exit' 1 2 3 15

$ trap 'rm tmp.$$' 0 # on program exit


You can't quote things reasonably in the csh:

set foo = "Bill asked, \"How's tricks?\""

doesn't work. This makes it really hard to construct strings with mixed quotes in them. In the Bourne shell, this works just fine. In fact, so does this:

cd /mnt; /usr/ucb/finger -m -s `ls \`u\``

Dollar signs cannot be escaped in double quotes in the csh. Ug.

set foo = "this is a \$dollar quoted and this is $HOME not quoted"
dollar: Undefined variable.

You have to use backslashes for newlines, and it's just darn hard to get them into strings sometimes.

set foo = "this \
and that";
echo $foo
this and that
echo "$foo"
Unmatched ".

Say what? You don't have these problems in the Bourne shell, where it's just fine to write things like this:

echo 'This is
some text that contains
several newlines.'


There's this big difference between global (environment) and local (shell) variables. In csh, you use a totally different syntax to set one from the other.

In Bourne shell, this

VAR=foo cmds args

is the same as

(export VAR; VAR=foo; cmd args)

or csh's

(setenv VAR; cmd args)

You can't use :t, :h, etc on envariables. Watch:

echo Try testing with $SHELL:t

It's really nice to be able to say




to be able to run the user's PAGER if set, and more otherwise. You can't do this in the csh. It takes more verbiage.

You can't get the process number of the last background command from the csh, something you might like to do if you're starting up several jobs in the background. In the Bourne shell, the pid of the last command put in the background is available in $!.


Consider this statement in the csh:


Despite your attempts to only set PAGER when you want to, the csh aborts:

MANPAGER: Undefined variable.

That's because it parses the whole line anyway and evaluates it! You have to write this:

if ($?MANPAGER) then

That's the same problem you have here:

if ($?X && $X == 'foo') echo ok
X: Undefined variable

This forces you to write a couple nested if statements. This is highly undesirable because it renders short-circuit booleans useless in situations like these. If the csh were the really C-like, you would expect to be able to safely employ this kind of logic. Consider the common C construct:

if (p && p->member)

Undefined variables are not fatal errors in the Bourne shell, so this issue does not arise there.

While the csh does have built-in expression handling, it's not what you might think. In fact, it's space sensitive. This is an error

@ a = 4/2

but this is ok

@ a = 4 / 2


Wouldn't it be nice to know you had an error in your script before you ran it? That's what the -n flag is for: just check the syntax. This is especially good to make sure seldom taken segments of code code are correct. Alas, the csh implementation of this doesn't work. Consider this statement:

exit (i)

Of course, they really meant

exit (1)

or just

exit 1

Either shell will complain about this. But if you hide this in an if clause, like so:

#!/bin/csh -fn
if (1) then
exit (i)

the csh tells you there's nothing wrong with this script. The equivalent construct in the Bourne shell, on the other hand, tells you this:

#!/bin/sh -n if (1) then exit (i) endif /tmp/x: syntax error at line 3: `(' unexpected


Here's one:

fg %?string
kill %?string
No match.

Huh? Here's another


Coredump, or garbage.

If you have an alias with backquotes, and use that in backquotes in another one, you get a coredump.

Try this:

% repeat 3 echo "/vmu*"


While some vendors have fixed some of the csh's bugs (the tcsh also does much better here), most of its problems can never be solved because they're a result of braindead design decisions. Do yourself a favor, and if you have to write a shell script, do it in the Bourne shell.